DESCRIPTION

This manual page describes the zlib general purpose compression library,
version 1.2.3.
The zlib compression library provides in-memory compression and
decompression functions, including integrity checks of the uncompressed
data. This version of the library supports only one compression method
(deflation) but other algorithms will be added later and will have the
same stream interface.
Compression can be done in a single step if the buffers are large enough
(for example if an input file is mmap'ed), or can be done by repeated
calls of the compression function. In the latter case, the application
must provide more input and/or consume the output (providing more output
space) before each call.
The compressed data format used by default by the in-memory functions is
the zlib format, which is a zlib wrapper documented in RFC 1950, wrapped
around a deflate stream, which is itself documented in RFC 1951.
The library also supports reading and writing files in gzip(1) (.gz) for-
mat with an interface similar to that of stdio(3) using the functions
that start with "gz". The gzip format is different from the zlib format.
gzip is a gzip wrapper, documented in RFC 1952, wrapped around a deflate
stream. This library can optionally read and write gzip streams in memory
as well.
The zlib format was designed to be compact and fast for use in memory and
on communications channels. The gzip format was designed for single-file
compression on file systems, has a larger header than zlib to maintain
directory information, and uses a different, slower, check method than
zlib.
The library does not install any signal handler. The decoder checks the
consistency of the compressed data, so the library should never crash
even in case of corrupted input.
The functions within the library are divided into the following sections:
- Basic functions
- Advanced functions
- Utility functions
- Checksum functions

BASIC FUNCTIONS

const char * zlibVersion(void);
The application can compare zlibVersion() and ZLIB_VERSION for
consistency. If the first character differs, the library code ac-
tually used is not compatible with the <zlib.h> header file used
by the application. This check is automatically made by defla-teInit() and inflateInit().
intdeflateInit(z_streamp strm, int level);
The deflateInit() function initializes the internal stream state
for compression. The fields zalloc, zfree, and opaque must be in-
itialized before by the caller. If zalloc and zfree are set to
Z_NULL, deflateInit() updates them to use default allocation
functions.
The compression level must be Z_DEFAULT_COMPRESSION, or between 0
and 9: 1 gives best speed, 9 gives best compression, 0 gives no
compression at all (the input data is simply copied a block at a
time).
Z_DEFAULT_COMPRESSION requests a default compromise between speed
and compression (currently equivalent to level 6).
deflateInit() returns Z_OK if successful, Z_MEM_ERROR if there
was not enough memory, Z_STREAM_ERROR if level is not a valid
compression level, Z_VERSION_ERROR if the zlib library version
(zlib_version) is incompatible with the version assumed by the
caller (ZLIB_VERSION). msg is set to null if there is no error
message. deflateInit() does not perform any compression: this
will be done by deflate().
intdeflate(z_streamp strm, int flush);
deflate() compresses as much data as possible, and stops when the
input buffer becomes empty or the output buffer becomes full. It
may introduce some output latency (reading input without
producing any output) except when forced to flush.
The detailed semantics are as follows. deflate() performs one or
both of the following actions:
Compress more input starting at next_in and update next_in and
avail_in accordingly. If not all input can be processed (because
there is not enough room in the output buffer), next_in and
avail_in are updated and processing will resume at this point for
the next call to deflate().
Provide more output starting at next_out and update next_out and
avail_out accordingly. This action is forced if the parameter
flush is non-zero. Forcing flush frequently degrades the compres-
sion ratio, so this parameter should be set only when necessary
(in interactive applications). Some output may be provided even
if flush is not set.
Before the call to deflate(), the application should ensure that
at least one of the actions is possible, by providing more input
and/or consuming more output, and updating avail_in or avail_out
accordingly; avail_out should never be zero before the call. The
application can consume the compressed output when it wants, for
example when the output buffer is full (avail_out == 0), or after
each call to deflate(). If deflate() returns Z_OK and with zero
avail_out, it must be called again after making room in the out-
put buffer because there might be more output pending.
Normally the parameter flush is set to Z_NO_FLUSH, which allows
deflate() to decide how much data to accumulate before producing
output, in order to maximise compression.
If the parameter flush is set to Z_SYNC_FLUSH, all pending output
is flushed to the output buffer and the output is aligned on a
byte boundary, so that the decompressor can get all input data
available so far. (In particular, avail_in is zero after the call
if enough output space has been provided before the call.) Flush-
ing may degrade compression for some compression algorithms and
so it should be used only when necessary.
If flush is set to Z_FULL_FLUSH, all output is flushed as with
Z_SYNC_FLUSH, and the compression state is reset so that
decompression can restart from this point if previous compressed
data has been damaged or if random access is desired. Using
Z_FULL_FLUSH too often can seriously degrade compression.
If deflate() returns with avail_out == 0, this function must be
called again with the same value of the flush parameter and more
output space (updated avail_out), until the flush is complete
(deflate() returns with non-zero avail_out). In the case of a
Z_FULL_FLUSH or a Z_SYNC_FLUSH, make sure that avail_out is
greater than six to avoid repeated flush markers due to avail_out
== 0 on return.
If the parameter flush is set to Z_FINISH, pending input is pro-
cessed, pending output is flushed and deflate() returns with
Z_STREAM_END if there was enough output space; if deflate() re-
turns with Z_OK, this function must be called again with Z_FINISH
and more output space (updated avail_out but no more input data,
until it returns with Z_STREAM_END or an error. After deflate()
has returned Z_STREAM_END, the only possible operations on the
stream are deflateReset() or deflateEnd().
Z_FINISH can be used immediately after deflateInit() if all the
compression is to be done in a single step. In this case,
avail_out must be at least the value returned by deflateBound()
(see below). If deflate() does not return Z_STREAM_END, then it
must be called again as described above.
deflate() sets strm->adler to the Adler-32 checksum of all input
read so far (that is, total_in bytes).
deflate() may update strm->data_type if it can make a good guess
about the input data type (Z_BINARY or Z_TEXT). If in doubt, the
data is considered binary. This field is only for information
purposes and does not affect the compression algorithm in any
manner.
deflate() returns Z_OK if some progress has been made (more input
processed or more output produced), Z_STREAM_END if all input has
been consumed and all output has been produced (only when flush
is set to Z_FINISH), Z_STREAM_ERROR if the stream state was in-
consistent (for example, if next_in or next_out was NULL),
Z_BUF_ERROR if no progress is possible (for example, avail_in or
avail_out was zero). Note that Z_BUF_ERROR is not fatal, and de-flate() can be called again with more input and more output space
to continue processing.
intdeflateEnd(z_streamp strm);
All dynamically allocated data structures for this stream are
freed. This function discards any unprocessed input and does not
flush any pending output.
deflateEnd() returns Z_OK if successful, Z_STREAM_ERROR if the
stream state was inconsistent, Z_DATA_ERROR if the stream was
freed prematurely (some input or output was discarded). In the
error case, msg may be set but then points to a static string
(which must not be deallocated).
intinflateInit(z_streamp strm);
The inflateInit() function initializes the internal stream state
for decompression. The fields next_in, avail_in, zalloc, zfree,
and opaque must be initialized before by the caller. If next_in
is not Z_NULL and avail_in is large enough (the exact value
depends on the compression method), inflateInit() determines the
compression method from the zlib header and allocates all data
structures accordingly; otherwise the allocation will be deferred
to the first call to inflate(). If zalloc and zfree are set to
Z_NULL, inflateInit() updates them to use default allocation
functions.
inflateInit() returns Z_OK if successful, Z_MEM_ERROR if there
was not enough memory, Z_VERSION_ERROR if the zlib library ver-
sion is incompatible with the version assumed by the caller. msg
is set to null if there is no error message. inflateInit() does
not perform any decompression apart from reading the zlib header
if present: this will be done by inflate(). (So next_in and
avail_in may be modified, but next_out and avail_out are un-
changed.)
intinflate(z_streamp strm, int flush);
inflate() decompresses as much data as possible, and stops when
the input buffer becomes empty or the output buffer becomes full.
It may introduce some output latency (reading input without
producing any output) except when forced to flush.
The detailed semantics are as follows. inflate() performs one or
both of the following actions:
Decompress more input starting at next_in and update next_in and
avail_in accordingly. If not all input can be processed (because
there is not enough room in the output buffer), next_in is updat-
ed and processing will resume at this point for the next call to
inflate().
Provide more output starting at next_out and update next_out and
avail_out accordingly. inflate() provides as much output as pos-
sible, until there is no more input data or no more space in the
output buffer (see below about the flush parameter).
Before the call to inflate(), the application should ensure that
at least one of the actions is possible, by providing more input
and/or consuming more output, and updating the next_* and avail_*
values accordingly. The application can consume the uncompressed
output when it wants, for example when the output buffer is full
(avail_out == 0), or after each call to inflate(). If inflate()
returns Z_OK and with zero avail_out, it must be called again
after making room in the output buffer because there might be
more output pending.
The flush parameter of inflate() can be Z_NO_FLUSH, Z_SYNC_FLUSH,
Z_FINISH, or Z_BLOCK. Z_SYNC_FLUSH requests that inflate() flush
as much output as possible to the output buffer. Z_BLOCK requests
that inflate() stop if and when it gets to the next deflate block
boundary. When decoding the zlib or gzip format, this will cause
inflate() to return immediately after the header and before the
first block. When doing a raw inflate, inflate() will go ahead
and process the first block, and will return when it gets to the
end of that block, or when it runs out of data.
The Z_BLOCK option assists in appending to or combining deflate
streams. Also to assist in this, on return inflate() will set
strm->data_type to the number of unused bits in the last byte
taken from strm->next_in, plus 64 if inflate() is currently
decoding the last block in the deflate stream, plus 128 if in-flate() returned immediately after decoding an end-of-block code
or decoding the complete header up to just before the first byte
of the deflate stream. The end-of-block will not be indicated un-
til all of the uncompressed data from that block has been written
to strm->next_out. The number of unused bits may in general be
greater than seven, except when bit 7 of data_type is set, in
which case the number of unused bits will be less than eight.
inflate() should normally be called until it returns Z_STREAM_END
or an error. However if all decompression is to be performed in a
single step (a single call to inflate), the parameter flush
should be set to Z_FINISH. In this case all pending input is pro-
cessed and all pending output is flushed; avail_out must be large
enough to hold all the uncompressed data. (The size of the un-
compressed data may have been saved by the compressor for this
purpose.) The next operation on this stream must be inflateEnd()
to deallocate the decompression state. The use of Z_FINISH is
never required, but can be used to inform inflate() that a faster
approach may be used for the single inflate() call.
In this implementation, inflate() always flushes as much output
as possible to the output buffer, and always uses the faster ap-
proach on the first call. So the only effect of the flush parame-
ter in this implementation is on the return value of inflate(),
as noted below, or when it returns early because Z_BLOCK is used.
If a preset dictionary is needed after this call (see infla-teSetDictionary() below), inflate() sets strm->adler to the
Adler-32 checksum of the dictionary chosen by the compressor and
returns Z_NEED_DICT; otherwise it sets strm->adler to the Adler-
32 checksum of all output produced so far (that is, total_out
bytes) and returns Z_OK, Z_STREAM_END or an error code as
described below. At the end of the stream, inflate() checks that
its computed Adler-32 checksum is equal to that saved by the
compressor and returns Z_STREAM_END only if the checksum is
correct.
inflate() will decompress and check either zlib-wrapped or gzip-
wrapped deflate data. The header type is detected automatically.
Any information contained in the gzip header is not retained, so
applications that need that information should instead use raw
inflate; see inflateInit2() below, or inflateBack() and perform
their own processing of the gzip header and trailer.
inflate() returns Z_OK if some progress has been made (more input
processed or more output produced), Z_STREAM_END if the end of
the compressed data has been reached and all uncompressed output
has been produced, Z_NEED_DICT if a preset dictionary is needed
at this point, Z_DATA_ERROR if the input data was corrupted (in-
put stream not conforming to the zlib format or incorrect check
value), Z_STREAM_ERROR if the stream structure was inconsistent
(for example, if next_in or next_out was NULL), Z_MEM_ERROR if
there was not enough memory, Z_BUF_ERROR if no progress is possi-
ble or if there was not enough room in the output buffer when
Z_FINISH is used. Note that Z_BUF_ERROR is not fatal, and in-flate() can be called again with more input and more output space
to continue compressing. If Z_DATA_ERROR is returned, the appli-
cation may then call inflateSync() to look for a good compression
block if a partial recovery of the data is desired.
intinflateEnd(z_streamp strm);
All dynamically allocated data structures for this stream are
freed. This function discards any unprocessed input and does not
flush any pending output.
inflateEnd() returns Z_OK if successful, or Z_STREAM_ERROR if the
stream state was inconsistent. In the error case, msg may be set
but then points to a static string (which must not be
deallocated).

ADVANCED FUNCTIONS

The following functions are needed only in some special applications.
intdeflateInit2(z_streamp strm, int level, int method, int windowBits,
int memLevel, int strategy);
This is another version of deflateInit() with more compression
options. The fields next_in, zalloc, zfree, and opaque must be
initialized before by the caller.
The method parameter is the compression method. It must be
Z_DEFLATED in this version of the library.
The windowBits parameter is the base two logarithm of the window
size (the size of the history buffer). It should be in the range
8..15 for this version of the library. Larger values of this
parameter result in better compression at the expense of memory
usage. The default value is 15 if deflateInit() is used instead.
windowBits can also be -8..-15 for raw deflate. In this case,
-windowBits determines the window size. deflate() will then gen-
erate raw deflate data with no zlib header or trailer, and will
not compute an Adler-32 check value.
windowBits can also be greater than 15 for optional gzip encod-
ing. Add 16 to windowBits to write a simple gzip header and
trailer around the compressed data instead of a zlib wrapper. The
gzip header will have no file name, no extra data, no comment, no
modification time (set to zero), no header crc, and the operating
system will be set to 255 (unknown). If a gzip stream is being
written, strm->adler is a crc32 instead of an adler32.
The memLevel parameter specifies how much memory should be allo-
cated for the internal compression state. memLevel=1 uses minimum
memory but is slow and reduces compression ratio; memLevel=9 uses
maximum memory for optimal speed. The default value is 8. See
<zconf.h> for total memory usage as a function of windowBits and
memLevel.
The strategy parameter is used to tune the compression algorithm.
Use the value Z_DEFAULT_STRATEGY for normal data; Z_FILTERED for
data produced by a filter (or predictor); Z_HUFFMAN_ONLY to force
Huffman encoding only (no string match), or Z_RLE to limit match
distances to one (run-length encoding). Filtered data consists
mostly of small values with a somewhat random distribution. In
this case, the compression algorithm is tuned to compress them
better. The effect of Z_FILTERED is to force more Huffman coding
and less string matching; it is somewhat intermediate between
Z_DEFAULT and Z_HUFFMAN_ONLY. Z_RLE is designed to be almost as
fast as Z_HUFFMAN_ONLY, but gives better compression for PNG im-
age data. The strategy parameter only affects the compression ra-
tio but not the correctness of the compressed output, even if it
is not set appropriately. Z_FIXED prevents the use of dynamic
Huffman codes, allowing for a simpler decoder for special appli-
cations.
deflateInit2() returns Z_OK if successful, Z_MEM_ERROR if there
was not enough memory, Z_STREAM_ERROR if a parameter is invalid
(such as an invalid method). msg is set to null if there is no
error message. deflateInit2() does not perform any compression:
this will be done by deflate().
intdeflateSetDictionary(z_streamp strm, const Bytef *dictionary, uIntdictLength);
Initializes the compression dictionary from the given byte se-
quence without producing any compressed output. This function
must be called immediately after deflateInit(), deflateInit2(),
or deflateReset(), before any call to deflate(). The compressor
and decompressor must use exactly the same dictionary (see
inflateSetDictionary()).
The dictionary should consist of strings (byte sequences) that
are likely to be encountered later in the data to be compressed,
with the most commonly used strings preferably put towards the
end of the dictionary. Using a dictionary is most useful when the
data to be compressed is short and can be predicted with good ac-
curacy; the data can then be compressed better than with the de-
fault empty dictionary.
Depending on the size of the compression data structures selected
by deflateInit() or deflateInit2(), a part of the dictionary may
in effect be discarded, for example if the dictionary is larger
than the window size in deflate() or deflate2(). Thus the strings
most likely to be useful should be put at the end of the diction-
ary, not at the front. In addition, the current implementation of
deflate() will use at most the window size minus 262 bytes of the
provided dictionary.
Upon return of this function, strm->adler is set to the Adler-32
value of the dictionary; the decompressor may later use this
value to determine which dictionary has been used by the compres-
sor. (The Adler-32 value applies to the whole dictionary even if
only a subset of the dictionary is actually used by the compres-
sor.) If a raw deflate was requested, then the Adler-32 value is
not computed and strm->adler is not set.
deflateSetDictionary() returns Z_OK if successful, or
Z_STREAM_ERROR if a parameter is invalid (such as NULL
dictionary) or the stream state is inconsistent (for example if
deflate() has already been called for this stream or if the
compression method is bsort). deflateSetDictionary() does not
perform any compression: this will be done by deflate().
intdeflateCopy(z_streamp dest, z_streamp source);
The deflateCopy() function sets the destination stream as a com-
plete copy of the source stream.
This function can be useful when several compression strategies
will be tried, for example when there are several ways of pre-
processing the input data with a filter. The streams that will be
discarded should then be freed by calling deflateEnd(). Note that
deflateCopy() duplicates the internal compression state which can
be quite large, so this strategy is slow and can consume lots of
memory.
deflateCopy() returns Z_OK if successful, Z_MEM_ERROR if there
was not enough memory, Z_STREAM_ERROR if the source stream state
was inconsistent (such as zalloc being NULL). msg is left un-
changed in both source and destination.
intdeflateReset(z_streamp strm);
This function is equivalent to deflateEnd() followed by defla-teInit(), but does not free and reallocate all the internal
compression state. The stream will keep the same compression lev-
el and any other attributes that may have been set by deflateIn-it2().
deflateReset() returns Z_OK if successful, or Z_STREAM_ERROR if
the source stream state was inconsistent (such as zalloc or state
being NULL).
intdeflateParams(z_streamp strm, int level, int strategy);
The deflateParams() function dynamically updates the compression
level and compression strategy. The interpretation of level and
strategy is as in deflateInit2(). This can be used to switch
between compression and straight copy of the input data, or to
switch to a different kind of input data requiring a different
strategy. If the compression level is changed, the input avail-
able so far is compressed with the old level (and may be
flushed); the new level will take effect only at the next call to
deflate().
Before the call to deflateParams(), the stream state must be set
as for a call to deflate(), since the currently available input
may have to be compressed and flushed. In particular, strm-
>avail_out must be non-zero.
deflateParams() returns Z_OK if successful, Z_STREAM_ERROR if the
source stream state was inconsistent or if a parameter was in-
valid, or Z_BUF_ERROR if strm->avail_out was zero.
intdeflateTune(z_streamp strm, int good_length, int max_lazy, intnice_length, int max_chain)
Fine tune deflate()'s internal compression parameters. This
should only be used by someone who understands the algorithm used
by zlib's deflate for searching for the best matching string, and
even then only by the most fanatic optimizer trying to squeeze
out the last compressed bit for their specific input data. Read
the deflate.c source code for the meaning of the max_lazy,
good_length, nice_length, and max_chain parameters.
deflateTune() can be called after deflateInit() or deflateIn-it2(), and returns Z_OK on success, or Z_STREAM_ERROR for an in-
valid deflate stream.
uLongdeflateBound(z_streamp strm, uLong sourceLen)
deflateBound() returns an upper bound on the compressed size
after deflation of sourceLen bytes. It must be called after de-flateInit() or deflateInit2(). This would be used to allocate an
output buffer for deflation in a single pass, and so would be
called before deflate().
intdeflatePrime(z_streamp strm, int bits, int value)
deflatePrime() inserts bits in the deflate output stream. The in-
tent is that this function is used to start off the deflate out-
put with the bits leftover from a previous deflate stream when
appending to it. As such, this function can only be used for raw
deflate, and must be used before the first deflate() call after a
deflateInit2() or deflateReset(). bits must be less than or equal
to 16, and that many of the least significant bits of value will
be inserted in the output.
deflatePrime() returns Z_OK if successful, or Z_STREAM_ERROR if
the source stream state was inconsistent.
intdeflateSetHeader(z_streamp strm, gz_headerp head)
deflateSetHeader() provides gzip header information for when a
gzip stream is requested by deflateInit2(). deflateSetHeader()
may be called after deflateInit2() or deflateReset() and before
the first call of deflate(). The text, time, os, extra field,
name, and comment information in the provided gz_header structure
are written to the gzip header (xflag is ignored - the extra
flags are set according to the compression level). The caller
must assure that, if not Z_NULL, name and comment are terminated
with a zero byte, and that if extra is not Z_NULL, that extra_len
bytes are available there. If hcrc is true, a gzip header CRC is
included. Note that the current versions of the command-line ver-
sion of gzip(1) do not support header CRCs, and will report that
it is a "multi-part gzip file" and give up.
If deflateSetHeader() is not used, the default gzip header has
text false, the time set to zero, and os set to 255, with no ex-
tra, name, or comment fields. The gzip header is returned to the
default state by deflateReset().
deflateSetHeader() returns Z_OK if successful, or Z_STREAM_ERROR
if the source stream state was inconsistent.
intinflateInit2(z_streamp strm, int windowBits);
This is another version of inflateInit() with an extra parameter.
The fields next_in, avail_in, zalloc, zfree, and opaque must be
initialized before by the caller.
The windowBits parameter is the base two logarithm of the maximum
window size (the size of the history buffer). It should be in the
range 8..15 for this version of the library. The default value is
15 if inflateInit() is used instead. windowBits must be greater
than or equal to the windowBits value provided to deflateInit2()
while compressing, or it must be equal to 15 if deflateInit2()
was not used. If a compressed stream with a larger window size is
given as input, inflate() will return with the error code
Z_DATA_ERROR instead of trying to allocate a larger window.
windowBits can also be -8..-15 for raw inflate. In this case,
-windowBits determines the window size. inflate() will then pro-
cess raw deflate data, not looking for a zlib or gzip header, not
generating a check value, and not looking for any check values
for comparison at the end of the stream. This is for use with
other formats that use the deflate compressed data format such as
zip. Those formats provide their own check values. If a custom
format is developed using the raw deflate format for compressed
data, it is recommended that a check value such as an Adler-32 or
a crc32 be applied to the uncompressed data as is done in the
zlib, gzip, and zip formats. For most applications, the zlib for-
mat should be used as is. Note that comments above on the use in
deflateInit2() applies to the magnitude of windowBits.
windowBits can also be greater than 15 for optional gzip decod-
ing. Add 32 to windowBits to enable zlib and gzip decoding with
automatic header detection, or add 16 to decode only the gzip
format (the zlib format will return a Z_DATA_ERROR). If a gzip
stream is being decoded, strm->adler is a crc32 instead of an
adler32.
inflateInit2() returns Z_OK if successful, Z_MEM_ERROR if there
was not enough memory, Z_STREAM_ERROR if a parameter is invalid
(such as a null strm). msg is set to null if there is no error
message. inflateInit2() does not perform any decompression apart
from reading the zlib header if present: this will be done by in-flate(). (So next_in and avail_in may be modified, but next_out
and avail_out are unchanged.)
intinflateSetDictionary(z_streamp strm, const Bytef *dictionary, uIntdictLength);
Initializes the decompression dictionary from the given un-
compressed byte sequence. This function must be called immediate-
ly after a call to inflate() if that call returned Z_NEED_DICT.
The dictionary chosen by the compressor can be determined from
the Adler-32 value returned by that call to inflate(). The
compressor and decompressor must use exactly the same dictionary
(see deflateSetDictionary()). For raw inflate, this function can
be called immediately after inflateInit2() or inflateReset() and
before any call to inflate() to set the dictionary. The applica-
tion must ensure that the dictionary that was used for compres-
sion is provided.
inflateSetDictionary() returns Z_OK if successful, Z_STREAM_ERROR
if a parameter is invalid (such as NULL dictionary) or the stream
state is inconsistent, Z_DATA_ERROR if the given dictionary
doesn't match the expected one (incorrect Adler-32 value). infla-teSetDictionary() does not perform any decompression: this will
be done by subsequent calls of inflate().
intinflateSync(z_streamp strm);
Skips invalid compressed data until a full flush point (see above
the description of deflate() with Z_FULL_FLUSH) can be found, or
until all available input is skipped. No output is provided.
inflateSync() returns Z_OK if a full flush point has been found,
Z_BUF_ERROR if no more input was provided, Z_DATA_ERROR if no
flush point has been found, or Z_STREAM_ERROR if the stream
structure was inconsistent. In the success case, the application
may save the current value of total_in which indicates where
valid compressed data was found. In the error case, the applica-
tion may repeatedly call inflateSync(), providing more input each
time, until success or end of the input data.
intinflateCopy(z_streamp dest, z_streamp source)
Sets the destination stream as a complete copy of the source
stream.
This function can be useful when randomly accessing a large
stream. The first pass through the stream can periodically record
the inflate state, allowing restarting inflate at those points
when randomly accessing the stream.
inflateCopy() returns Z_OK if success, Z_MEM_ERROR if there was
not enough memory, Z_STREAM_ERROR if the source stream state was
inconsistent (such as zalloc being NULL). msg is left unchanged
in both source and dest.
intinflateReset(z_streamp strm);
This function is equivalent to inflateEnd() followed by infla-teInit(), but does not free and reallocate all the internal
decompression state. The stream will keep attributes that may
have been set by inflateInit2().
inflateReset() returns Z_OK if successful, or Z_STREAM_ERROR if
the source stream state was inconsistent (such as zalloc or state
being NULL).
intinflatePrime(z_stream strm, int bits, int value)
This function inserts bits in the inflate input stream. The in-
tent is that this function is used to start inflating at a bit
position in the middle of a byte. The provided bits will be used
before any bytes are used from next_in. This function should only
be used with raw inflate, and should be used before the first in-flate() call after inflateInit2() or inflateReset(). bits must be
less than or equal to 16, and that many of the least significant
bits of value will be inserted in the input.
inflatePrime() returns Z_OK if successful, or Z_STREAM_ERROR if
the source stream state was inconsistent.
intinflateGetHeader(z_streamp strm, gz_headerp head)
inflateGetHeader() requests that gzip header information be
stored in the provided gz_header structure. inflateGetHeader()
may be called after inflateInit2() or inflateReset(), and before
the first call of inflate(). As inflate() processes the gzip
stream, head->done is zero until the header is completed, at
which time head->done is set to one. If a zlib stream is being
decoded, then head->done is set to -1 to indicate that there will
be no gzip header information forthcoming. Note that Z_BLOCK can
be used to force inflate() to return immediately after header
processing is complete and before any actual data is
decompressed.
The text, time, xflags, and os fields are filled in with the gzip
header contents. hcrc is set to true if there is a header CRC.
(The header CRC was valid if done is set to one.) If extra is not
Z_NULL, then extra_max contains the maximum number of bytes to
write to extra. Once done is true, extra_len contains the actual
extra field length, and extra contains the extra field, or that
field truncated if extra_max is less than extra_len. If name is
not Z_NULL, then up to name_max characters are written there,
terminated with a zero unless the length is greater than
name_max. If comment is not Z_NULL, then up to comm_max charac-
ters are written there, terminated with a zero unless the length
is greater than comm_max. When any of extra, name, or comment are
not Z_NULL and the respective field is not present in the header,
then that field is set to Z_NULL to signal its absence. This al-
lows the use of deflateSetHeader() with the returned structure to
duplicate the header. However if those fields are set to allocat-
ed memory, then the application will need to save those pointers
elsewhere so that they can be eventually freed.
If inflateGetHeader() is not used, then the header information is
simply discarded. The header is always checked for validity, in-
cluding the header CRC if present. inflateReset() will reset the
process to discard the header information. The application would
need to call inflateGetHeader() again to retrieve the header from
the next gzip stream.
inflateGetHeader() returns Z_OK if successful, or Z_STREAM_ERROR
if the source stream state was inconsistent.
intinflateBackInit(z_stream *strm, int windowBits, unsigned char FAR
*window)
Initialize the internal stream state for decompression using in-flateBack() calls. The fields zalloc, zfree and opaque in strm
must be initialized before the call. If zalloc and zfree are
Z_NULL, then the default library-derived memory allocation rou-
tines are used. windowBits is the base two logarithm of the win-
dow size, in the range 8..15. window is a caller supplied buffer
of that size. Except for special applications where it is assured
that deflate() was used with small window sizes, windowBits must
be 15 and a 32K byte window must be supplied to be able to
decompress general deflate streams.
See inflateBack() for the usage of these routines.
inflateBackInit() will return Z_OK on success, Z_STREAM_ERROR if
any of the parameters are invalid, Z_MEM_ERROR if the internal
state could not be allocated, or Z_VERSION_ERROR if the version
of the library does not match the version of the header file.
intinflateBack(z_stream *strm, in_func in, void FAR *in_desc, out_funcout, void FAR *out_desc)
inflateBack() does a raw inflate with a single call using a
call-back interface for input and output. This is more efficient
than inflate() for file I/O applications in that it avoids copy-
ing between the output and the sliding window by simply making
the window itself the output buffer. This function trusts the ap-
plication to not change the output buffer passed by the output
function, at least until inflateBack() returns.
inflateBackInit() must be called first to allocate the internal
state and to initialize the state with the user-provided window
buffer. inflateBack() may then be used multiple times to inflate
a complete, raw deflate stream with each call. inflateBackEnd()
is then called to free the allocated state.
A raw deflate stream is one with no zlib or gzip header or
trailer. This routine would normally be used in a utility that
reads zip or gzip files and writes out uncompressed files. The
utility would decode the header and process the trailer on its
own, hence this routine expects only the raw deflate stream to
decompress. This is different from the normal behavior of in-flate(), which expects either a zlib or gzip header and trailer
around the deflate stream.
inflateBack() uses two subroutines supplied by the caller that
are then called by inflateBack() for input and output. inflate-Back() calls those routines until it reads a complete deflate
stream and writes out all of the uncompressed data, or until it
encounters an error. The function's parameters and return types
are defined above in the in_func and out_func typedefs. inflate-Back() will call in(in_desc, &buf) which should return the number
of bytes of provided input, and a pointer to that input in buf.
If there is no input available, in() must return zero - buf is
ignored in that case - and inflateBack() will return a buffer er-
ror. inflateBack() will call out(out_desc, buf, len) to write the
uncompressed data buf[0..len-1]. out() should return zero on suc-
cess, or non-zero on failure. If out() returns non-zero, inflate-Back() will return with an error. Neither in() nor out() are per-
mitted to change the contents of the window provided to infla-teBackInit(), which is also the buffer that out() uses to write
from. The length written by out() will be at most the window
size. Any non-zero amount of input may be provided by in().
For convenience, inflateBack() can be provided input on the first
call by setting strm->next_in and strm->avail_in. If that input
is exhausted, then in() will be called. Therefore strm->next_in
must be initialized before calling inflateBack(). If strm-
>next_in is Z_NULL, then in() will be called immediately for in-
put. If strm->next_in is not Z_NULL, then strm->avail_in must
also be initialized, and then if strm->avail_in is not zero, in-
put will initially be taken from strm->next_in[0 .. strm-
>avail_in - 1].
The in_desc and out_desc parameters of inflateBack() are passed
as the first parameter of in() and out() respectively when they
are called. These descriptors can be optionally used to pass any
information that the caller-supplied in() and out() functions
need to do their job.
On return, inflateBack() will set strm->next_in and strm-
>avail_in to pass back any unused input that was provided by the
last in() call. The return values of inflateBack() can be
Z_STREAM_END on success, Z_BUF_ERROR if in() or out() returned an
error, Z_DATA_ERROR if there was a format error in the deflate
stream (in which case strm->msg is set to indicate the nature of
the error), or Z_STREAM_ERROR if the stream was not properly ini-
tialized. In the case of Z_BUF_ERROR, an input or output error
can be distinguished using strm->next_in which will be Z_NULL
only if in() returned an error. If strm->next is not Z_NULL, then
the Z_BUF_ERROR was due to out() returning non-zero. (in() will
always be called before out(), so strm->next_in is assured to be
defined if out() returns non-zero.) Note that inflateBack() can-
not return Z_OK.
intinflateBackEnd(z_stream *strm)
All memory allocated by inflateBackInit() is freed.
inflateBackEnd() returns Z_OK on success, or Z_STREAM_ERROR if
the stream state was inconsistent.
uLongzlibCompileFlags(void)
This function returns flags indicating compile-time options.
Type sizes, two bits each:
00 16 bits
01 32 bits
10 64 bits
11 other:
1.0 size of uInt
3.2 size of uLong
5.4 size of voidpf (pointer)
7.6 size of z_off_t
Compiler, assembler, and debug options:
8 DEBUG
9 ASMV or ASMINF - use ASM code
10 ZLIB_WINAPI - exported functions use the WINAPI
calling convention
11 0 (reserved)
One-time table building (smaller code, but not thread-safe if
true):
12 BUILDFIXED -- build static block decoding tables
when needed
13 DYNAMIC_CRC_TABLE -- build CRC calculation tables
when needed
14,15 0 (reserved)
Library content (indicates missing functionality):
16 NO_GZCOMPRESS - gz* functions cannot compress (to
avoid linking deflate code when not needed)
17 NO_GZIP - deflate can't write gzip streams, and in-
flate can't detect and decode gzip streams (to
avoid linking CRC code)
18-19 0 (reserved)
Operation variations (changes in library functionality):
20 PKZIP_BUG_WORKAROUND - slightly more permissive in-
flate
21 FASTEST - deflate algorithm with only one, lowest
compression level
22,23 0 (reserved)
The sprintf variant used by gzprintf (zero is best):
24 0 = vs*, 1 = s* - 1 means limited to 20 arguments
after the format
25 0 = *nprintf, 1 = *printf - 1 means gzprintf() not
secure!
26 0 = returns value, 1 = void - 1 means inferred
string length returned
Remainder:
27-31 0 (reserved)

UTILITY FUNCTIONS

The following utility functions are implemented on top of the basic
stream-oriented functions. To simplify the interface, some default op-
tions are assumed (compression level and memory usage, standard memory
allocation functions). The source code of these utility functions can
easily be modified if you need special options.
intcompress(Bytef *dest, uLongf *destLen, const Bytef *source, uLongsourceLen);
The compress() function compresses the source buffer into the
destination buffer. sourceLen is the byte length of the source
buffer. Upon entry, destLen is the total size of the destination
buffer, which must be at least the value returned by
compressBound(sourcelen). Upon exit, destLen is the actual size
of the compressed buffer. This function can be used to compress a
whole file at once if the input file is mmap'ed.
compress() returns Z_OK if successful, Z_MEM_ERROR if there was
not enough memory, or Z_BUF_ERROR if there was not enough room in
the output buffer.
intcompress2(Bytef *dest, uLongf *destLen, const Bytef *source, uLongsourceLen, int level);
The compress2() function compresses the source buffer into the
destination buffer. The level parameter has the same meaning as
in deflateInit(). sourceLen is the byte length of the source
buffer. Upon entry, destLen is the total size of the destination
buffer, which must be at least the value returned by
compressBound(sourceLen). Upon exit, destLen is the actual size
of the compressed buffer.
compress2() returns Z_OK if successful, Z_MEM_ERROR if there was
not enough memory, Z_BUF_ERROR if there was not enough room in
the output buffer, or Z_STREAM_ERROR if the level parameter is
invalid.
intcompressBound(uLong sourceLen)
compressBound() returns an upper bound on the compressed size
after compress() or compress2() on sourceLen bytes. It would be
used before a compress() or compress2() call to allocate the des-
tination buffer.
intuncompress(Bytef *dest, uLongf *destLen, const Bytef *source, uLongsourceLen);
The uncompress() function decompresses the source buffer into the
destination buffer. sourceLen is the byte length of the source
buffer. Upon entry, destLen is the total size of the destination
buffer, which must be large enough to hold the entire un-
compressed data. (The size of the uncompressed data must have
been saved previously by the compressor and transmitted to the
decompressor by some mechanism outside the scope of this compres-
sion library.) Upon exit, destLen is the actual size of the
compressed buffer. This function can be used to decompress a
whole file at once if the input file is mmap'ed.
uncompress() returns Z_OK if successful, Z_MEM_ERROR if there was
not enough memory, Z_BUF_ERROR if there was not enough room in
the output buffer, or Z_DATA_ERROR if the input data was corrupt-
ed or incomplete.
gzFilegzopen(const char *path, const char *mode);
The gzopen() function opens a gzip (.gz) file for reading or
writing. The mode parameter is as in fopen(3) ("rb" or "wb") but
can also include a compression level (wb9) or a strategy: 'f' for
filtered data, as in "wb6f"; 'h' for Huffman only compression, as
in "wb1h", or 'R' for run-length encoding as in "wb1R". (See the
description of deflateInit2() for more information about the
strategy parameter.)
gzopen() can be used to read a file which is not in gzip format;
in this case gzread() will directly read from the file without
decompression.
gzopen() returns NULL if the file could not be opened or if there
was insufficient memory to allocate the (de)compression state;
errno can be checked to distinguish the two cases (if errno is
zero, the zlib error is Z_MEM_ERROR).
gzFilegzdopen(int fd, const char *mode);
The gzdopen() function associates a gzFile with the file descrip-
tor fd. File descriptors are obtained from calls like open(2),
dup(2), creat(3), pipe(2), or fileno(3) (if the file has been
previously opened with fopen(3)). The mode parameter is as in
gzopen().
The next call to gzclose() on the returned gzFile will also close
the file descriptor fd, just like fclose(fdopen(fd), mode) closes
the file descriptor fd. If you want to keep fd open, use
gzdopen(dup(fd), mode).
gzdopen() returns NULL if there was insufficient memory to allo-
cate the (de)compression state.
intgzsetparams(gzFile file, int level, int strategy);
The gzsetparams() function dynamically updates the compression
level or strategy. See the description of deflateInit2() for the
meaning of these parameters.
gzsetparams() returns Z_OK if successful, or Z_STREAM_ERROR if
the file was not opened for writing.
intgzread(gzFile file, voidp buf, unsigned len);
The gzread() function reads the given number of uncompressed
bytes from the compressed file. If the input file was not in gzip
format, gzread() copies the given number of bytes into the
buffer.
gzread() returns the number of uncompressed bytes actually read
(0 for end of file, -1 for error).
intgzwrite(gzFile file, voidpc buf, unsigned len);
The gzwrite() function writes the given number of uncompressed
bytes into the compressed file. gzwrite() returns the number of
uncompressed bytes actually written (0 in case of error).
intgzprintf(gzFile file, const char *format, ...);
The gzprintf() function converts, formats, and writes the args to
the compressed file under control of the format string, as in
fprintf(3). gzprintf() returns the number of uncompressed bytes
actually written (0 in case of error). The number of uncompressed
bytes written is limited to 4095. The caller should make sure
that this limit is not exceeded. If it is exceeded, then
gzprintf() will return an error (0) with nothing written. In this
case, there may also be a buffer overflow with unpredictable
consequences, which is possible only if zlib was compiled with
the insecure functions sprintf() or vsprintf() because the secure
snprintf() or vsnprintf() functions were not available.
intgzputs(gzFile file, const char *s);
The gzputs() function writes the given null-terminated string to
the compressed file, excluding the terminating null character.
gzputs() returns the number of characters written, or -1 in case
of error.
char * gzgets(gzFile file, char *buf, int len);
The gzgets() function reads bytes from the compressed file until
len-1 characters are read, or a newline character is read and
transferred to buf, or an end-of-file condition is encountered.
The string is then terminated with a null character.
gzgets() returns buf, or Z_NULL in case of error.
intgzputc(gzFile file, int c);
The gzputc() function writes c, converted to an unsigned char,
into the compressed file. gzputc() returns the value that was
written, or -1 in case of error.
intgzgetc(gzFile file);
The gzgetc() function reads one byte from the compressed file.
gzgetc() returns this byte or -1 in case of end of file or error.
intgzungetc(int c, gzFile file)
Push one character back onto the stream to be read again later.
Only one character of push-back is allowed. gzungetc() returns
the character pushed, or -1 on failure. gzungetc() will fail if a
character has been pushed but not read yet, or if c is -1. The
pushed character will be discarded if the stream is repositioned
with gzseek() or gzrewind().
intgzflush(gzFile file, int flush);
The gzflush() function flushes all pending output into the
compressed file. The parameter flush is as in the deflate() func-
tion. The return value is the zlib error number (see function
gzerror() below). gzflush() returns Z_OK if the flush parameter
is Z_FINISH and all output could be flushed.
gzflush() should be called only when strictly necessary because
it can degrade compression.
z_off_tgzseek(gzFile file, z_off_t offset, int whence);
Sets the starting position for the next gzread() or gzwrite() on
the given compressed file. The offset represents a number of
bytes in the uncompressed data stream. The whence parameter is
defined as in lseek(2); the value SEEK_END is not supported.
If the file is opened for reading, this function is emulated but
can be extremely slow. If the file is opened for writing, only
forward seeks are supported; gzseek() then compresses a sequence
of zeroes up to the new starting position.
gzseek() returns the resulting offset location as measured in
bytes from the beginning of the uncompressed stream, or -1 in
case of error, in particular if the file is opened for writing
and the new starting position would be before the current posi-
tion.
intgzrewind(gzFile file);
The gzrewind() function rewinds the given file. This function is
supported only for reading.
gzrewind(file) is equivalent to (int)gzseek(file, 0L, SEEK_SET).
z_off_tgztell(gzFile file);
The gztell() function returns the starting position for the next
gzread() or gzwrite() on the given compressed file. This position
represents a number of bytes in the uncompressed data stream.
gztell(file) is equivalent to gzseek(file, 0L, SEEK_CUR).
intgzeof(gzFile file);
The gzeof() function returns 1 when EOF has previously been
detected reading the given input stream, otherwise zero.
intgzdirect(gzFile file);
The gzdirect() function returns 1 if the file is being read
directly without compression; otherwise it returns 0.
intgzclose(gzFile file);
The gzclose() function flushes all pending output if necessary,
closes the compressed file and deallocates all the
(de)compression state. The return value is the zlib error number
(see function gzerror() below).
const char * gzerror(gzFile file, int *errnum);
The gzerror() function returns the error message for the last er-
ror which occurred on the given compressed file. errnum is set to
the zlib error number. If an error occurred in the file system
and not in the compression library, errnum is set to Z_ERRNO and
the application may consult errno to get the exact error code.
voidgzclearerr(gzFile file)
Clears the error and end-of-file flags for file. This is analo-
gous to the clearerr() function in stdio. This is useful for con-
tinuing to read a gzip file that is being written concurrently.

CHECKSUM FUNCTIONS

These functions are not related to compression but are exported anyway
because they might be useful in applications using the compression li-
brary.
uLongadler32(uLong adler, const Bytef *buf, uInt len);
The adler32() function updates a running Adler-32 checksum with
the bytes buf[0..len-1] and returns the updated checksum. If buf
is NULL, this function returns the required initial value for the
checksum.
An Adler-32 checksum is almost as reliable as a CRC32 but can be
computed much faster. Usage example:
uLong adler = adler32(0L, Z_NULL, 0);
while (read_buffer(buffer, length) != EOF) {
adler = adler32(adler, buffer, length);
}
if (adler != original_adler) error();
uLongadler32_combine(uLong adler1, uLong adler2, z_off_t len2)
The adler32_combine() function combines two Adler-32 checksums
into one. For two sequences of bytes, seq1 and seq2 with lengths
len1 and len2, Adler-32 checksums are calculated for each, adler1
and adler2. adler32_combine() returns the Adler-32 checksum of
seq1 and seq2 concatenated, requiring only adler1, adler2, and
len2.
uLongcrc32(uLong crc, const Bytef *buf, uInt len);
The crc32() function updates a running CRC-32 with the bytes
buf[0..len-1] and returns the updated CRC-32. If buf is NULL,
this function returns the required initial value for the CRC.
Pre- and post-conditioning (one's complement) is performed within
this function so it shouldn't be done by the application. Usage
example:
uLong crc = crc32(0L, Z_NULL, 0);
while (read_buffer(buffer, length) != EOF) {
crc = crc32(crc, buffer, length);
}
if (crc != original_crc) error();
uLongcrc32_combine(uLong crc1, uLong crc2, z_off_t len2)
The crc32_combine() function combines two CRC-32 check values
into one. For two sequences of bytes, seq1 and seq2 with lengths
len1 and len2, CRC-32 check values are calculated for each, crc1
and crc2. crc32_combine() returns the CRC-32 check value of seq1
and seq2 concatenated, requiring only crc1, crc2, and len2.

STRUCTURES

struct internal_state;
typedef struct z_stream_s {
Bytef *next_in; /* next input byte */
uInt avail_in; /* number of bytes available at next_in */
off_t total_in; /* total nb of input bytes read so far */
Bytef *next_out; /* next output byte should be put there */
uInt avail_out; /* remaining free space at next_out */
off_t total_out; /* total nb of bytes output so far */
char *msg; /* last error message, NULL if no error */
struct internal_state FAR *state; /* not visible by applications */
alloc_func zalloc; /* used to allocate the internal state */
free_func zfree; /* used to free the internal state */
voidpf opaque; /* private data object passed to zalloc and zfree*/
int data_type; /* best guess about the data type: binary or text*/
uLong adler; /* adler32 value of the uncompressed data */
uLong reserved; /* reserved for future use */
} z_stream;
typedef z_stream FAR * z_streamp;
/*
gzip header information passed to and from zlib routines.
See RFC 1952 for more details on the meanings of these fields.
*/
typedef struct gz_header_s {
int text; /* true if compressed data believed to be text */
uLong time; /* modification time */
int xflags; /*extra flags (not used when writing a gzip file)*/
int os; /* operating system */
Bytef *extra; /* pointer to extra field or Z_NULL if none */
uInt extra_len; /* extra field length (valid if extra != Z_NULL) */
uInt extra_max; /* space at extra (only when reading header) */
Bytef *name; /* pointer to zero-terminated file name or Z_NULL*/
uInt name_max; /* space at name (only when reading header) */
Bytef *comment; /* pointer to zero-terminated comment or Z_NULL */
uInt comm_max; /* space at comment (only when reading header) */
int hcrc; /* true if there was or will be a header crc */
int done; /* true when done reading gzip header (not used
when writing a gzip file) */
} gz_header;
typedef gz_header FAR *gz_headerp;
The application must update next_in and avail_in when avail_in has
dropped to zero. It must update next_out and avail_out when avail_out has
dropped to zero. The application must initialize zalloc, zfree, and
opaque before calling the init function. All other fields are set by the
compression library and must not be updated by the application.
The opaque value provided by the application will be passed as the first
parameter for calls to zalloc() and zfree(). This can be useful for cus-
tom memory management. The compression library attaches no meaning to the
opaque value.
zalloc must return Z_NULL if there is not enough memory for the object.
If zlib is used in a multi-threaded application, zalloc and zfree must be
thread safe.
On 16-bit systems, the functions zalloc and zfree must be able to allo-
cate exactly 65536 bytes, and more (for Unix).
The fields total_in and total_out can be used for statistics or progress
reports. After compression, total_in holds the total size of the un-
compressed data and may be saved for use in the decompressor (particular-
ly if the decompressor wants to decompress everything in a single step).